In the use of holography to record digital data, a number of systems employ arrays of lenslets, often referred to as "fly's eye lenses". Examples of these uses are found in U.S. Pat. Nos. 3,765,749, 3,533,673 and 3,716,287. In U.S. Pat. No. 3,765,749 two fly's eye lenses are described that have focal ratios given at f/3.6. The fly's eye lens of 3,533,673 constitutes an array of spherical lenslets, again of very small f- number although no specific number is given. U.S. Pat. No. 3,716,287, assigned to the assignee of the present invention, describes the use of such a lens in a particular type of data recorder in which an array of light modulators is used. In this system, a fly's eye lens is employed for focusing the beams from the modulator array in such fashion as to introduce phase randomization that averages out excessive intensity variations.
While the system of U.S. Pat. No. 3,716,287 has been constructed and operates fully satisfactorily as described, it is limited in efficiency by practical considerations. Commercially available lens arrays of non-prohibitive cost comprise, for example, embossed plastic elements having small f- numbers (e.g. f/3), and relatively good lens quality with adequate phase randomization between lenslets. Only by rare coincidence, however, would such an available lens meet the specifications for a particular holographic system, in which the number and spacing of elements and the f- number are determined by other considerations. Of course, the necessary tooling for making an array for a specific application can be manufactured, but at far greater cost. Even if arrays can be produced that have suitable numbers of lenslets with sufficiently small spacings between the lenslets, certain optical limitations cannot be avoided. Specifically, it is highly unlikely that high f- numbers can be supplied, that leakage between lenslets can be minimized without loss of intensity or that high optical efficiency can be achieved. In addition, pits, bubbles or other defects in the individual lenslets can render an entire array unsuitable for use.
In some systems it is known to utilize a light diverting or diffracting element in step and repeat fashion to successively record a number of holograms in a matrix pattern. Each of these holograms may thereafter be used for a comparable light diverting or diffracting function. In U.S. Pat. No. 3,744,871, for example, a successive recording technique is used for preparing what is termed an "ordinary-illumination hologram" for purposes of beam redirection when used in a light modulator system. While the description is not clear as to the mechanism involved, it is evident that a point diffraction technique is involved in deflecting the beam and that it is then brought down to a point for recording at each position. However, both the point diffraction and the point recording are particularly subject to the effects of impurities, grain defects, bubbles and other film and optical imperfections that inevitably are encountered, because such imperfections are relatively large in comparison to the small areas used. In addition the efficiency with which the light available for illumination is used is very low. Thus, this approach is far less than adequate for present day systems, in which each element of a high density matrix must meet high standards as to optical characteristics and signal-to-noise ratio. Furthermore, a hologram prepared in this manner requires additional optical elements in order to precisely illuminate each element of a light modulator array.
U.S. Pat. No. 3,941,450 contains a later teaching of the usage of a two-dimensional diffraction grating structure for what is termed a matrix of holographic lenses. This may be provided by a step and repeat technique (FIG. 5) or by simultaneous recording (FIG. 6), because of the lengthy and delicate character of the former type of recording. It is evident, however, that the simultaneous process disclosed also has severe deficiencies, since it is prepared by using a diffraction pattern comprising a matrix of openings of preselected size and pitch. Furthermore, while the pitch must be small the size must be very small, in order to obtain a suitable diffraction characteristic. Consequently, the matrix must itself be very precisely prepared, using repeating techniques that approach the limits of feasibility. Even so, the system remains subject to the limitations and problems previously mentioned, because the minute matrix openings create high sensitivity to dust, size variations and other effects, because available light is used with low efficiency, and because only divergent beams can be generated so that other optics are needed for most practical applications.